The present invention relates to an ultrasonic method for inspecting tubular members for internal corrosion and particularly to a method for displaying the results of the inspection. The above referenced patent discloses an ultrasonic inspection tool of the pulse-echo type having a probe adapted to be moved through the tubular member. The probe includes an ultrasonic transducer which produces ultrasonic waves that are directed normally to the wall of the tubular member by a rotating 45-degree mirror. The transducer is energized to transmit an acoustic pulse and then acts as a receiver to receive the echoes or reflections of the acoustic pulse from the inner and outer walls of the tubular member. The referenced patent also discloses the measuring of the time interval between the reflection of the acoustic energy from the inner and outer walls of the tubular member. The time intervals are then segregated according to their magnitude by a distribution analyser and recorded on separate recorders, the number of counts in each recorder being proportional to the area having a thickness in the range shown for the recorder. The presence of a large number of short time intervals would indicate extensive thinning of the tube wall by corrosion. If all of the time intervals fell within a relatively narrow range, it would indicate uniform wall thickness and the absence of any corrosion.
The above recording system has several problems which reduced the effectiveness of the inspection tool. For example, the rough inner wall of a corroded tube scatters and attenuates the ultrasonic pulse so that the pulse reflected from the outer wall may be too small to be detected. In this event, the wall thickness measurement is missed. A rough inner wall can also cause multiple reflections from the inner wall. Since the interval between these reflected pulses is not related to the wall thickness, they produce false measurements of the wall thickness. Finally, overlap of the outer wall reflection by the much larger inner wall reflection prevents measuring walls thinner than about 0.05 inches. Because the more severely corroded and rougher areas produce a larger percentage of missed measurements, the measurements that are made are biased in favor of the smoother and less corroded areas. Furthermore, the false measurements obscure the valid measurements, especially in the low range of measurable thickness. These problems, combined with the limit on the minimum measurable thickness, make it impossible to make a reliable assessment of the extent and severity of the corrosion damage.
These problems are largely avoided in the present invention.